1. Field Of The Invention
This invention relates to stabilized oscillators operating in the microwave frequency range, and in particular, to a circuit for turning such devices off and on efficiently, without the necessity of having to de-energize the circuit entirely and without the use of isolating switches which must be relatively large to achieve the degree of isolation required.
2. Summary of the Prior Art
Oscillator circuits are used widely in the electronics field for a variety of purposes. Oscillators operating within the microwave region have come to be commonly utilized in the fields of radar and telecommunications, in which the oscillators are employed as sub-components in timing or distance-measuring functions, e.g., local oscillators.
It is well known in the art that a negative resistance oscillator can be created by adding the proper choice of terminating impedences to an unstable, active element.
At microwave frequencies, it is common to employ a gallium arsenide field effect transistor (GaAs FET) as the active element of the oscillator circuit in order to take advantage of the high switching speeds of these devices.
An additional requirement placed on oscillators, particularly those operating in timing and measuring applications, is that of frequency stability. In the 1-20 GHz region, it is not uncommon to employ a resonator having a resonant cavity filled with a dielectric material electrically coupled into the oscillator circuit. If such a resonator is suitably configured to have a very large Q, the frequency stability of the overall oscillator circuit will become primarily a function of the stability of the electrical parameters of the resonator itself and independent of other time-varying parameters in the balance of the circuit. Moreover, if sufficiently large values of Q for the resonator are selected, the effect of the incorporation of the resonator into the circuit is to drive the frequency of the oscillator strongly in the direction necessary to achieve zero phase shift in the tuned circuit, i.e., to restore the drifting frequency of the device to the desired, nominal frequency.
A dielectric-resonator-stabilized oscillator (DRSO) employing a GaAs FET device as the active element of the oscillator in combination with an improved, laser-tunable dielectric resonator coupled into the circuit by means of a stripline transmission line and operating at frequencies in the region of 1-20 GHz with frequency stabilities of 1-5 ppm/.degree.C. is the subject of a co-pending application, Ser. No. 752,297, filed July 5, 1985, and assigned to the same assignee as herein.
In the field in which such circuits as these find application, it is frequently necessary to be able rapidly to switch the oscillator on and off, i.e., to "blank" the oscillator's output signal for brief, periodic increments of time, or it may be desirable to modulate the output of the oscillator.
In the prior art, this was accomplished by removing the biasing voltage from the transistor in the oscillator, which effectively turned the transistor off. However, this method creates frequency stability problems caused by thermal transitions within the components of the oscillator, particularly the transistor, because the power has been turned off. When power is reapplied to the circuit to turn it on, final frequency stability is not achieved for short periods of time until thermal equilibrium is reestablished.
Similarly, placing switches at the output of the oscillator circuit can create isolation problems in that, at microwave frequencies, a simple switch may appear as a closed circuit because of capacitance effects. While microwave switches are known in the art which are capable of achieving as much as 60 db of isolation, their bulky size and expense may preclude their use in some applications.